PHP - Pasar un xml por php

'<?xml version="1.0" standalone="no"?>

	<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

			<soapenv:Header>

					<vodh:VODHeader xmlns:vodh="http://www.vodafone.com/soap/header/">

							<vodh:commandId>ServiceDelivery</vodh:commandId>

							<vodh:authentication>

									<vodh:username>xxxx</vodh:username>

									<vodh:password>xxxxxxx</vodh:password>

							</vodh:authentication>

							<vodh:service>

									<vodh:serviceID>xxxxxx</vodh:serviceID>

									<vodh:serviceType>SMS</vodh:serviceType>

							</vodh:service>

					</vodh:VODHeader>

			</soapenv:Header>

			<soapenv:Body>

					<vodb:VODBody xmlns:vodb="http://www.vodafone.com/soap/body/" version="1.0">

							<vodb:contextID></vodb:contextID>

							<vodb:destAddress>666666666</vodb:destAddress>

							<vodb:subServiceId>123456</vodb:subServiceId>

							<vodb:messageBody>texto de prueba</vodb:messageBody>

							<vodb:bodyIsText>true</vodb:bodyIsText>

							<vodb:deliveryReport>false</vodb:deliveryReport>

							<vodb:priorityFlag>0</vodb:priorityFlag>

							<vodb:dataCodingScheme>0</vodb:dataCodingScheme>

							<vodb:sourceTON>0</vodb:sourceTON>

							<vodb:destTON>1</vodb:destTON>

							<vodb:sourceNPI>0</vodb:sourceNPI>

							<vodb:destNPI>1</vodb:destNPI>

							<vodb:esmClass>0</vodb:esmClass>

							<vodb:protocolId>0</vodb:protocolId>

					</vodb:VODBody>

			</soapenv:Body>

	</soapenv:Envelope>'

class autorizacion {

        public $username;

        public $password;

        public function __construct($usuario, $pass) {

            $this->username = $usuario;

            $this->password = $pass;

        }

    }

    class servicio {

        public $serviceID;

        public $serviceType;

        public function __construct($serviceID, $serviceType) {

            $this->serviceID = $serviceID;

            $this->serviceType = $serviceType;

        }

    }

    $usuario = "xxxxx";

    $pass ="xxxxxxx";

    $url = "http://serv.prep.cdm.vodafone.es/soap/SOAPSMS";

    $cliente = new SoapClient($url, array("trace" => 1, "exception" => 0));

    // crear cabecera

    $autorizar = new autorizacion($usuario, $pass);

    $servicio = new servicio("xxxxxxxxxx", "SMS");

    $cabecera[] = new SoapHeader("http://www.w3.org/2001/XMLSchema-instance", "commandId", "ServiceDelivery", FALSE);

    $cabecera[] = new SoapHeader("http://www.w3.org/2001/XMLSchema-instance", "authentication", $autorizar, FALSE);

    $cabecera[] = new SoapHeader("http://www.w3.org/2001/XMLSchema-instance/", "service", $servicio, FALSE);